The handling of a multiphase fluid, that is, a mixture of at least two fluids of different phases, presents problems arising for example from the different physical characteristics of liquids and gases, in particular, the virtual incompressibility of the former and the ready compressibility of the latter, and also from variations in the relative amounts of liquids and gases in the multiphase fluid. For example, in oil production, a well may produce a mixture of crude oil, crude gas, water and sand or like particulate material. It is desirable in many instances to place such a mixture under increased pressure, but this is difficult because pumps with impellers designed to pump liquid are unsuitable where the liquid contains a high gas content. Similarly, ordinary gas compressors are unsuitable for use where liquid is present in the gas in any substantial amount.